This invention relates in general to charge transfer devices and in particular to such devices which include a plurality of charge transfer channels or shift registers on a common substrate for providing different time delays simultaneously variable to a plurality of sampled analog signals.
In certain applications such as in ultrasound imaging apparatus it is necessary to provide a plurality of different time delays to signals to enable coherent summation of the signals. In the case of the ultrasound imaging apparatus these signals are received from a transducer array. It is also necessary to simultaneously vary the time delays provided by the channels to each signal to effect a scanning of the field of view by the receiving beam of the transducer array. Each transducer of the array receives the ultrasound echo originating from an image point in the field of view at a slightly different time so that each channel or shift register requires a different compensation delay. The relationship of the required steering delays for the various shift registers is normally linear for steering the beam to an angle from the array normal. In this case, a linear variation of the delay of the various shift registers provides the proper delay for all the channels. Charge transfer channels or shift registers formed on a common substrate have been utilized for providing the various delay compensations required to effect the coherent summation. In particular, frequency dependent charge transfer devices which provide frequency dependent delays have been used. One form of such charge transfer device is shown and described in connection with FIG. 9 of an article entitled "Ultrasonic Imaging Using Two-Dimensional Transducer Arrays" by W. L. Beaver et al, appearing in "Cardio-Vascular Imaging and Image Processing," Theory and Practice, 1975, Volume 72, published by the Society of Photo-Optical Instrumentation Engineers, Palos Verdes Estates, Calif. In FIG. 9 of this article is shown a plurality of signal processing channels for processing echo signals from an array of ultrasonic transducers. Each of the channels includes a first and a second CCD delay line for steering. The first delay lines vary in length linearly from one side of the array to the other and the second delay lines also vary in length in a complementary fashion. A first clock provides voltages of first frequencies to the first delay lines and a second clock provides voltages of second frequencies to the second delay lines. The total number of stages in the first and second delay lines of each channel is the same. Thus, the relative delays of the channels and hence the angle of the receiving beam formed is dependent on the relative frequencies of the first and second clocks. If these frequencies are the same, the receiving beam would be directed along a normal to the array. When the frequency of the first clock is at one extreme value and the frequency of the second clock is at an opposite extreme value the receiving beam is directed along one extreme angle in respect to the normal of the array and vica versa. When a large number of channels or shift registers are required to be formed on a common substrate corresponding to the utilization of a large number of transducers in an array and hence the utilization of a large number of signal processing channels, a large number of CCD shift registers are required to be formed on a common substrate and also a large number of electrical contacts or terminals must be provided therefor. The fabrication of such devices or chips with high yield is difficult.